1. Field of the Invention
This invention relates generally to microwave frequency dielectric resonator oscillators, and relates more particularly to a parallel-feedback, dielectric resonator oscillator that generates microwave energy at a selected one of several fixed frequencies.
2. Description of the Relevant Art
Microwave frequency oscillators are key elements of many communication systems and radars. A fixed frequency microwave oscillator is often implemented in a hybrid circuit using an amplifier and a parallel feedback path comprised of a dielectric resonator and associated microstrip lines. One parallel-feedback, dielectric resonator oscillator, known in the prior art, is shown in FIG. 1. Other, series-feedback, dielectric resonator oscillators are shown in co-pending patent application Ser. No. 809,161, which is hereby incorporated by reference.
The prior art oscillator of FIG. 1 includes a dielectric resonator 10 as a parallel feedback element coupled between the input and output terminals of an amplifier 12 via a feedback microstrip line 14. The output terminal of the amplifier is coupled to the load 18 via an output microstrip line 20. The dielectric resonator 10 is disposed between the output and feedback microstrip lines 20 and 14, and acts to couple some of the output energy of the amplifier, at a resonant frequency of the dielectric resonator, back into the input terminal of the amplifier. The amplifier typically contains a field-effect or bipolar transistor plus the appropriate biasing circuitry. In operation, the amplifier 12 oscillates at a frequency equal to a resonant frequency of the dielectric resonator 10.
While such single frequency oscillators are useful, there is also a need for microwave oscillators that can selectively generate any of several discrete frequencies. The design requirements of such an oscillator include (1) output frequency selection from several available frequencies, (2) stable output frequencies, (3) fast switching between frequencies, and (4) no spurious signals.
One approach to providing a multiple-frequency oscillator is to join together several dielectric resonator oscillators, as shown in FIG. 2. Any one of three separate dielectric resonator oscillators 22, 24, and 26, each with a different operational frequency, are selectively connected to a load 28 via a switch 30. The switch 30, which is shown schematically as a single-pole, triple-throw switch, may be implemented using PIN type diodes.
In some implementations of such a multiple frequency device, all of the dielectric resonator oscillators would continuously operate in order to provide stable operation and to allow fast switching from one frequency to another. In theory, only the signal generated by the selected dielectric resonator oscillator is supplied to the output terminal 32. In actuality, however, signals from the non-selected dielectric resonator oscillators leak through the switch to create unwanted spurious signals in the output signal. Extremely high isolation switches are required to reduce such signal leakage. The isolation values required are generally difficult to meet as a practical matter even with complex and expensive multi-throw switches, particularly within the X and Ku frequency bands. The presence of spurious signals can be a very severe problem in certain electronic warfare systems, wherein a spurious signal may be erroneously interpreted as a threat signal.
In other implementations of prior art multiple frequency oscillators, the dielectric resonator oscillators are switched on only when needed to generate the output signal. While this approach eliminates the spurious signal problem, it greatly increases the switching time because one oscillator must be switched on and another oscillator must be switched off each time the output frequency is changed. In addition, frequency of the output signal may wander somewhat before the selected oscillator stabilizes. Such implementations, therefore, suffer from the drawbacks of increased switching time and decreased stability.